The biologic significance of reverse tri-iodothyronine is not yet identified. Cord blood values reflect the fetal thyroid state, with rT 3 excess and tri-iodothyronine deficiency. In an attempt to correlate thyroid function and gestational age, T 3 and rT 3 concentrations were measured by radioimmunoassay on 64 cord blood samples. Infant gestational ages ranged from 29 to 42 weeks; birth weights from 650 to 3,870 gm. The results show a significant positive correlation between gestational age and T 3 and a significant negative correlation between gestational age and rT 3. The rT 3/T 3 offers the best correlation.